Needing to overcome a 1-0 first-leg deficit, the Reds thumped Gala 4-1 to reach the Champions League quarterfinals.
Zenden said on BBC Radio: "It’s one of those spectacular nights that you can expect on a Champion’s League evening, the fans were obviously more than good for them yesterday...Anfield on a European night can be magical.
"Arne Slot needed a bit of a result and also a good performance once again to prove he’s the right man, I think up until the end of the season I think the pressure will stay on but I think it shows he can still manage the team and turn things around.
"Defensively they’ve lacked a bit… and also up front, when you create 10 chances, of course you need to put away at least 4 of them, but if they don’t, they’re always likely to concede on the other end and that’s what happened too often this season.
"It was a pity to see him (Mohamed Salah) shoot the penalty like he did, I think we’ve never seen him take a penalty like he did yesterday, but it was quickly forgotten… to score a typical Mo Salah goal, his 50th goal in the European completion, the first African player to do so is something to take as a very positive.
"Salah has a lot in his locker and that’s why he’s still at Liverpool and wants to prove himself."